Measuring particulate matter in a fluid

ABSTRACT

A device for measuring near forward scatter caused by particulate matter in a fluid is disclosed. The device comprises, according to various embodiments, a transceiver and a reflector. The transceiver includes a light source and a detector. The reflector is positioned opposite the transceiver so that at least a portion of the fluid is present between the transceiver and the reflector. The reflector includes a front portion facing the transceiver. Optical energy from the light source incident upon the reflector is reflected towards the transceiver as a return beam of optical energy, wherein the detector senses primarily optical energy of the return beam that is scattered over a range of near forward angles by particulate matter in the fluid.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of co-pending U.S. application Ser.No. 10/857,548 filed May 28, 2004, which is incorporated herein byreference.

BACKGROUND

The present invention is directed generally to the measurement ofcertain optical properties of the interaction of light with particulatematter in a fluid.

Power generation plants and other industrial facilities may releaseparticulate matter into the atmosphere, especially if the facilitiesburn coal or other fossil fuels. Particulate matter in the flue gasesgenerated from the burning of coal tends to be mostly spherical,solidified droplets of a variety of sizes from the metallic componentsof the ash in the coal. Because the particulate matter degrades airquality and reduces visibility (haze) govemmental regulations requiresome facilities to continuously control and monitor their particulatematter emissions. Particulate mass emission rates are reported in unitsof weight per time (tons/yr), and are determined from particulate massloading measurements in wt/volume (grains/standard cubic foot) and acorresponding measure of stack gas flow rate in volume/time (standardcubic feet/hour). For example, Environmental Protection Agency (EPA) orstate regulations require some sources to continuously monitor the massloading of their particulate matter emissions to prove compliance withparticulate emission limitations.

Although a number of techniques are available to measure the massloading of particulate matter emissions directly, many are costly andincapable of reliable continuous operation under field conditions.Therefore, continuous indirect measurements of optical or otherproperties related to the mass loading are often used in associationwith a correlation coefficient or a transfer function that allowscalculation and output of an equivalent particulate mass loading value.Recently, the EPA implemented Performance Specification 11 (40 C.F.R. §60, Appendix B, PS-11) to define how a particulate monitor is to betested, and the resulting performance that is required to “certify” aparticulate monitor. Certification is a term designating that a specificmonitor meets the relevant EPA performance requirements for that type ofanalyzer and can be used to report emissions for determination of thecompliance status of a given emission source. PS-11 requires thatproperties of particulate matter measured by continuous techniques becorrelated with, or converted to, particulate mass loading and comparedto a reference method (40 C.F.R. § 60, Appendix A, RM 5 or 17, forexample) measurement, or series of measurements. Reference methodmeasurements for particulate mass loading use a manual stack samplingtechnique and are non-continuous measurements. Several differentcharacteristics of the interaction of light with particulate matter canbe measured and correlated with particulate density, including thosediscussed below.

Extinction is a measure of the quantity of optical energy absorbed orscattered by particulate matter. Extinction may be measured over manywavelengths of light, however, some extinction measurements, calledopacity measurements, require light spanning the visible range (400 to700 nm). Extinction measurements are dependent on variables other thanmass loading, thus reducing the accuracy of an extinction measurement asa predictor of particulate mass loading. For example, extinction isdependent on particulate shape and size, with the strongest response(for a given mass loading) occurring when particulate diameter iscomparable to the wavelength of the light source. Given constantparticle size distribution, shape and specific gravity, the extinctionresponse is closely correlated with mass loading. Extinctionmeasurements may be unusable at very low particulate mass loadings asthe extinction measurement becomes difficult to make with good accuracy.The specific gravity of the particulate matter itself also affects thecorrelation of the extinction measurement with particulate mass loadingbecause the optical properties of the particulate is caused by thesurface area of the particulates and not the material inside theparticles.

FIG. 1 depicts a prior art device 100 for measuring extinction caused byparticulate matter 128 in a fluid 116, which may be an exhaust stream inan exhaust stack 120. In the device 100, a light source 102 directs aforward light beam 112 through a beam splitter 106 and lens 108 toward areflector 110. The reflector 110 receives the forward beam 112 andreflects a return beam 114 toward a detector 104 after reflecting thereturn beam 114 off of a beam splitter 106. The beams 112, 114 propagateacross the fluid 116 through holes 130, 132 in the stack 120. A largeportion of the optical energy present in the beams 112, 114 reaches thedetector 104; however, some optical energy is absorbed or scattered bythe particulate matter 128 present in the fluid 116. The differencebetween the optical energy emitted by the light source 102 and detectedby the detector 104 with a clear path (no particulate in this case) andthe optical energy received by the detector 104 with particulate presentin the optical path indicates the extinction caused by the particulate,thereby providing an indication of the amount of the particulate 128present in the fluid 116. Since the beam goes across the stack and back,these optical systems are also known as double-pass.

Scatter is a measure of the quantity of optical energy that is scatteredby particulate matter, or reflected off the axis of the interrogatinglight beam. Devices for measuring scatter may direct a light beam acrossan exhaust stream and measure the quantity of light that is scattered atdifferent angles away from the beam's expected propagation direction.Backscatter refers to scatter near 180 degrees away from the projectedbeam, and forward scatter refers to scatter near the same angle as theprojected beam. Different types of scatter measurements detected at avariety of angles, light source wavelengths and beam widths displaydifferent degrees of dependence on particulate properties other thanmass loading.

Backscatter, for example, measures optical energy reflected byparticulate matter in a backward direction compared to the direction ofthe projected beam. Backscatter is somewhat less dependent onparticulate size than extinction, but still responds most strongly tovery small particles. Also, unlike extinction, backscatter provides astrong response when particulate mass loading is low. Like extinction,however, backscatter techniques are very sensitive to the size, andshape of the particulate. Backscatter is also sensitive to particulatecolor.

FIG. 2 depicts a prior art device 200 for measuring backscatter in thefluid 216. In the device 200, the light source 202 directs a forwardbeam 212 through a beam splitter 206 and lens 208 across the exhauststack 220 through holes 230, 232. An absorbing device 218, i.e. anoptically black material, positioned opposite the light source 202prevents the forward beam 212 from being reflected back to the detector.Even though the forward beam 212 is not reflected by the absorbingdevice 218, optical energy from the forward beam 212 is still scatteredback to the detector 204 by particulate 228 present in the fluid 216after reflecting off of beam splitter 206. The amount of optical energyreceived by the detector 204 is indicative of the backscatter caused bythe particulate, thereby providing an indicator of the quantity of theparticulate 228 in the fluid 216.

Near angle forward scatter is a measurement of the optical energy thatis scattered by particulate matter in small, near forward angles. Likebackscatter, near forward scatter provides a strong response whenparticulate mass loading is low. Unlike extinction and backscatter,however, near forward scatter provides a strong response for relativelylarge particulates with diameters greater than 2 microns with a visiblelight source. In addition, near forward scatter techniques minimize theundesirable color, size, and shape dependencies of extinction andbackscatter measurements.

Existing devices for measuring near forward scatter are oftencomplicated, expensive, and difficult to calibrate and maintain. Somerequire beam steering as well as active optical equipment on both sidesof the exhaust stream. Many are point sampling or close coupledextractive devices and hence introduce measurement errors inapplications with significant particulate stratification. Existingdevices are also unable to take advantage of the strengths of otherknown techniques, for example extinction and backscatter. Consequently,there exists a need for a simple, efficient way to measure near angleforward scatter. In addition, there exists a need for a technique toimplement multiple measurement methods with a single device so that thecorrelation of a group of optical measurements with particulate densitycan be made more independent of the size, shape, and color of theparticulate matter itself than can be obtained with a singlemeasurement.

SUMMARY OF THE INVENTION

In one general respect, the present invention is directed to a devicefor measuring near forward scatter caused by particulate matter in afluid. The device, according to various embodiments, comprises atransceiver and a reflector. The transceiver includes a light source anda detector. The light source may be selected depending on thewavelengths of light to be measured and the size of the particulate,which in combination may determine the efficiency of the scatteringprocess at any particular angle. Further a variety of light sources withdifferent wavelengths may be used to more accurately determine theproperties of the particulate. The reflector is positioned opposite thetransceiver so that at least a portion of the fluid is present betweenthe transceiver and the reflector. The reflector includes a frontportion facing the transceiver. Optical energy from the light sourceincident upon the reflector is reflected towards the transceiver as areturn beam of optical energy that is shaped such that ideally none ofthe main projected beam impinges directly on the detector. The lightreaching the detector may be primarily the light that is scattered fromthe returned beam over a range of near forward angles by particulatematter in the fluid.

According to various implementations, the range of near forward anglesmay be between 1 and 5 degrees. Also, the front portion of the reflectormay include a variety of lenses, such as a convex or concave sphericallens, a concave or convex wedge shaped lens, such wedge shaped lensesalso being known as conical prisms or conical lenses. The lens may alsoinclude a central, non-transmissive portion,

In addition, the device may include means for replacing the lens of thereflector with a second, or third, differently shaped lens. Also, thedevice may include means for replacing the reflector with a secondreflector. The second reflector may be similar to the first, but thefront portion or size may be different. Also, in variousimplementations, the return beam from the reflector may cross itself inthe field of view of the detector.

The device may be mounted on an exhaust stack and, as such, may be formeasuring particulate matter present in an exhaust stream in the exhauststack. Alternatively, the device may be mounted in a diversion chamber,which may be an external measurement chamber fed from the exhaust stackbut may be separate from it.

In another general aspect, the present invention is directed to a systemfor monitoring particulate matter in a fluid. According to variousembodiments, the system may include a transceiver and a reflectorassembly. The reflector assembly may be positioned opposite thetransceiver so that at least a portion of the fluid is present betweenthe transceiver and the reflector assembly. Further, the reflectorassembly may include a first reflector, including a front portion facingthe transceiver, such that, when the first reflector is in the path ofthe light beam from the light source of the transceiver, optical energyfrom the light source incident upon the reflector is reflected towardsthe transceiver as a return beam of optical energy, wherein primarilyoptical energy of the return beam that is scattered over a range of nearforward angles by particulate matter in the fluid reaches the detector.Additionally, the reflector assembly may include a second optical deviceand means for cyclically moving the first reflector and the secondoptical device into the path of the light beam from the light source.

In various implementations, the second optical device may comprise anextinction reflector for taking extinction measurements and/or anabsorbing device for taking backscatter measurements. The system mayfurther comprise a correlation module in communication with the detectorand the reflector assembly for correlating detected optical signals fromthe detector with particulate mass loading and/or size of theparticulate matter in the fluid.

In yet another general aspect, the present invention is directed to amethod of measuring mass loading of particulate matter in a fluid. Themethod may include, according to various embodiments, measuring nearforward scatter caused by particulate matter in the fluid as well asmeasuring a second property of the particulate matter in the fluid. Thesecond property may be, for example, backscatter or extinction caused bythe particulate matter in the fluid. The method may further comprisecalculating the mass loading of the particulate matter in the fluidbased on the near forward scatter and the second and/or a third propertyalong with other gas or process measurements. Further, there may bemultiple measurements of the forward and/or backscatter measurementscorresponding to different scattering angles and light sources withdifferent predominant wavelengths.

DESCRIPTION OF THE FIGURES

Embodiments of the present invention will be described by way of examplein conjunction with the following Figures, wherein:

FIG. 1 is a diagram of a prior art device for measuring extinction;

FIG. 2 is a diagram of a prior art device for measuring back scatter;

FIGS. 3-6 are diagrams of devices for measuring near forward scatteraccording to various embodiments of the present invention;

FIGS. 7-8 and 11-13 are diagrams of various components of systems formeasuring particulate matter according to various embodiments of thepresent invention; and

FIGS. 9-10 are flow charts of methods for correlating variousparticulate matter measurements to a desired property according tovarious embodiments of the present invention.

DESCRIPTION OF THE INVENTION

FIGS. 3, 3A and 4 depict a device 300 for measuring near forward scattercaused by particulates 328 in a fluid 316 according to variousembodiments of the present invention. The device 300 may include atransceiver 301 and a reflector 310. The transceiver 301 may include alight source 302, a detector 304, a beam splitter 306 and a lens 308.The light source 302 may be any device capable of projecting a lightbeam such as, for example, a light emitting diode (LED), a diode or gaslaser, or an incandescent source. According to one embodiment, a greenLED may be used as the light source 302. The wavelength of the lightsource 302 may be determined based on the size of the particulates to bemeasured. Also, according to various aspects, the device 300 may containmore than one light source to measure scattered optical energy atmultiple wavelengths. The detector 304 may be a photodiode or any otherdevice capable of sensing optical energy reflected from the reflector310 at a small off-axis angle and scattered by the particulate in adirection toward the lens 308 and detector 304 by the beam splitter 306.

The reflector 310 may be, for example, a total internal reflecting glasscorner cube or any other device suitable for reflecting light. It willbe appreciated that a corner cube has the unique property that itreturns the incoming beam parallel to but displaced from the incomingbeam. Therefore, using a corner cube as the reflector 310 may make thedevice 300 much less sensitive to small variations in alignment betweenthe transceiver and reflector. The reflector 310 may also include afront portion 318. The front portion 318 may be, a wedge prism, asdepicted in FIGS. 3 and 4, however the front portion 318 could assumeother shapes according to other embodiments as described below. Invarious embodiments, the device 300 may be mounted on an exhaust stack320 where the fluid 316 is part of an exhaust stream.

The device 300 may measure near forward scatter by directing a forwardlight beam 312 from the light source 302 toward the beam splitter 306and lens 308, which may direct the light beam toward the reflector 310though holes 330, 332 in the stack 320. The reflector 310 may reflect areturn beam 314 in the general direction of the transceiver 301. Inparticular, the front portion 318 of the reflector 310 may shape thereturn beam 314 so that substantially all of the optical energy reachingthe detector 304 has been scattered at a near forward angle byparticulate 328 present in the fluid 316. In other words, but for lightscattered by particulate 328 at near forward angles, no return opticalenergy would ideally reach the detector 304. For example, the frontportion 318 of the reflector 310 may shape the return beam 314 into ahollow cone, with the hollow portion of the cone falling such thatoptical energy propagating in the direction of the return beam 314 doesnot ordinarily reach the detector 304. Optical energy scattered byparticulate present in the fluid 316 at a near forward angle, though,may reach the detector 304, as shown in FIG. 3A.

FIG. 3A is an illustration of near forward scatter by particulate 328using the device 300 of FIG. 3. The arrow 326 represents a photonpropagating in the direction of the return beam 314. The photon 326 maycome into contact with particulate 328. When the conditions for nearforward scatter exist, the photon 326 may be deflected, or scattered, bythe particulate 328 to a new propagation path differing from that of thereturn beam 314 by a near forward angle 334. Depending on the nearforward angle 334, the photon 326 may strike the lens 308 and ultimatelythe detector 304. The detector 304 may be able to sense photons 326,that is, optical energy, scattered from the path of the return beam 314at a range of near forward angles 334. In various embodiments, the rangeof near forward angles 334 visible to the detector 304 may be between 1and 5 degrees, though any range of near forward angles may be used.

In the device 300, the front portion 318 of the reflector 310 may be alens 402. The lens 402 may be a convex wedge shaped lens, or a conicalprism, as shown in FIG. 4, and may modify the return light beam 314 toform two contiguous cones, 406 and 408 which are adjoined by their apexangles. The cone 408 may be hollow, preventing un-scattered opticalenergy of the return beam 314 from reaching the detector 304. Scatteredoptical energy from the return beam 314, however, may reach the detector304 when it is scattered over the range of scattering angles 334 visibleto the detector 304. The range of scattering angles 334 visible to thedetector 304 may be dependent on the shape of the cones 406 and 408 andmay be modified by changing the wedge angle of the lens 402. Also,because the cones 406 and 408 cross over each other within the field ofview of the detector 304, the quantity of scattered optical energyvisible to the detector 304 may be maximized.

According to other various embodiments of the present invention, thefront portion 318 of the reflector 310 may be of various other shapes.For example, FIG. 5 is a diagram of the device 300 having a differentconfiguration for the front portion 318 of the reflector 310 than inFIG. 4. In FIG. 5, the front portion 318 of the reflector 310 includes aconcave wedge shaped lens 502, which may modify the shape of the returnbeam 314, as shown in FIG. 5, causing it to form one hollow cone 504.FIG. 5 shows that the hollow portion of the cone 504 falls around thelens 308. Thus, un-scattered optical energy from the return beam 314 maybe prevented from reaching the detector 304. Optical energy from thereturn beam 314, however, may reach the detector 304 if it is scatteredby particulate matter (not shown) in the fluid 316 over a range ofscattering angles 334 visible to the detector 304. The range ofscattering angles 334 visible to the detector 304 may be related to theshape of the cone 504, and may be modified by changing the wedge angleof lens 502.

In some conditions, optical energy may be reflected by the lens 402 or502 directly towards the transceiver 301. Also, if the apex of lens 402or 502 and/or the apex of the reflector 310 are not both aligned withthe optical axis of the transceiver 301, a cone of un-scattered lightmay be directed toward the transceiver 301. As such, the transceiver 301may be shielded from extraneous un-scattered optical energy by creatinga black or otherwise non-transmissive portion (not shown) on the frontportion of the lens 402 or 502. In this way, near forward anglescattered optical energy detected by the detector 304 will not beoverwhelmed by un-scattered optical energy directed towards thetransceiver 301.

According to other various embodiments of the present invention, thefront portion 318 of the reflector 310 may assume yet other shapes. FIG.6 is a diagram of the device 300 having a different configuration forthe front portion 318 of the reflector 310 than in FIG. 4 or FIG. 5. InFIG. 6, the front portion 318 of the reflector 310 includes a convexspherical lens 602. According to various embodiments, the convex lens602 modifies the shape of the return beam 314, as shown in FIG. 6,causing it to form two adjoining cones of light 604 and 606. In someembodiments, the front portion of the lens 602 may include a black orotherwise non-transmissive portion 608, which may cause the cone 606 tobe hollow, blocking the optical energy that would otherwise have fallenon lens 308 and ultimately the detector 304. This may preventun-scattered optical energy of the return beam 314 from reaching thedetector 304. Thus the primary optical energy falling on the detector304 may be that which has been scattered from the return beam 314 by arange of near forward angles 334. The range of near forward scatteringangles 334 visible to the detector 304 may be modified by changing thefocal length of lens 602, or the size and location of the opaque mark608.

The device 300, according to various embodiments, is an improvement overthe devices 100, 200 above because it makes a near forward scattermeasurement using many of the same components as a conventionaldouble-pass extinction measurement instrument, thereby providingsubstantial cost savings and the ability to use the same transceiver forextinction and for forward scatter and backscatter measurements. Asdiscussed above in more detail, near forward scatter provides a strongerresponse than extinction when particulate mass loading is low, andprovides a stronger response than back-scatter or extinction forrelatively large particulates. In addition, near forward scatter is lessdependent on color and size than either of these methods. The device 300is also an improvement over existing devices for measuring near forwardscatter because it involves fewer moving parts, and is easier tocalibrate. For example, some existing near forward scatter measuringdevices require a beam steering apparatus. Not only does this add movingparts to the device, but it also makes calibration more complex.

Various embodiments of the present invention are also directed to asystem 700 for taking various different measurements of particulatematter in a fluid 116, as shown in FIGS. 7 and 8. The system 700 maytake measurements using multiple techniques over ranges, for example, ofoptical wavelengths, and scattering angles. The system 700 is similar tothe devices 300 of FIGS. 4-6, except that the system 700 includes areflector assembly 704 that permits the measurement of near forwardscatter of the particulate matter in the fluid 116, as well as othermeasurements of particulate matter in a fluid, including, for example,extinction and/or backscatter measurements. In order to achieve this,the reflector assembly 704, according to various embodiments, maycomprise a wheel 706 having different front portions (lenses) that arepositioned in front of a reflector 714, or having different reflectorswith front portions thereon including at least one near forward scatterreflector 310, such as different ones of the reflectors 310 shown inFIGS. 4-6, as well as an extinction reflector 110, such as describedabove in conjunction with FIG. 1, and/or a backscatter absorbing device218, such as described above in conjunction with FIG. 2. In FIG. 8,three near forward scatter reflectors or front portions thereof 310_(a-c) are shown, although according to various embodiments a differentquantity of different near forward scatter reflectors 310 could beincluded on the wheel 706. The near forward reflectors 310 _(a-c) mayall be different from one another, such as variations on the reflectorsshown in FIGS. 4-6, to thereby provide different measurements. Forexample, the near forward reflectors 310 _(a-c) may be configured toallow the measurement of near forward scatter over different ranges ofscattering angles. The reflector assembly 704 may include a steppermotor 708 or other electromechanical positioning device for rotating thewheel 706. In various other embodiments, the wheel 706 may have variousoptical devices, for example, a front portion 318, a backscatterabsorbing device 218, or no device at all. In these various embodiments,rotating the wheel may cause the various optical devices to be placed infront of a stationary reflector 310.

In operation, as the wheel 706 rotates, the various reflectors 310, 110and/or absorbing device 218 are cyclically rotated into the path of thebeam from the transceiver 301. The detector 304 may detect the variousamounts of optical energy incident on the transceiver 301 and within theangle of view of the detector for each different type of reflector 310,110 or absorbing device 218. In this way, measurements to determine themass loading of particulate in the fluid may be collected for nearforward scattering measurement techniques, as well as for extinctionand/or backscattering techniques. The combination of data from thesedifferent measurements techniques may provide an even more accurateindication of the mass loading of particulate in the fluid.Additionally, combining near forward scatter measurements taken overdifferent ranges of scattering angles and with different light sourcesmay give an improved indication of particulate size and mass loading.

In addition to the components described above, the system 700 mayinclude a computing device 710 in communication with the detector 304 ofthe transceiver 301 and the reflector assembly 704. The detected lightmeasurements from the detector 304 for the various reflectors/absorbersof the reflector assembly 704 may be communicated to the computingdevice 710, which may also receive timing information from the reflectorassembly 704 to synchronize the various amounts of detected light fromthe detector 304 to the reflector 310, 110 or absorbing device 218 inthe path of the beam at the time. Based on this information, thecomputing device 710 may, for example, correlate extinction, backscatterand multiple near angle forward scatter optical measurements withparticulate mass loading.

The computing device 710 may include, for example, one or a number ofnetworked personal computers, laptops, servers, microprocessors,micro-controllers, etc. The computing device 710 may also include acorrelation module 712 for performing the correlation calculations. Thecorrelation module 712 may be implemented as software code to beexecuted by a processor (not shown) of the computing device 710 usingany type of computer instruction type suitable such as, for example,Java, C, C++, Visual Basic, etc., using, for example, conventional orobject-oriented techniques. The software code may be stored as a seriesof instructions or commands on a computer readable medium, such as arandom access memory (RAM), a read only memory (ROM), a magnetic mediumsuch as a hard-drive or a floppy disk, or an optical medium such as aCD-ROM.

The correlation module 712 may convert the results of one or moremeasurements into a calculated value of mass loading based on acorrelated model. Equation 1, below, describes a model for correlatingmeasurements of various particulate matter properties to mass loading,or another desired property that the correlation module 712 may employto perform the correlation according to various embodiments.y(x ₁ , x ₂ , x ₃ , . . . x _(n))=a ₁ *x ₁ +a ₂ *x ₂ +a ₃ *x ₃ + . . .+a _(n) *x _(n)  (1)In equation 1, the function y may represent the mass loading predictionof the model. The dependent variables x₁-x_(n) may representmeasurements of particular properties of particulate matter. Forexample, x₁ may represent an extinction or opacity measurement, and x₂may represent a backscatter measurement. The variables x₃ through XN mayrepresent near angle forward scatter measurements. According to variousembodiments, different near angle forward scatter measurements may betaken using different configurations of the system 700. Also, differentnear angle forward scatter measurements may measure near angle forwardscatter over different ranges of scattering angles.

Still referring to equation 1, the variables a₁-a_(n) may representcorrelation coefficients for dependent variables x₁-x_(n). For example,a₁ may be the correlation coefficient for an extinction measurement, a₂may be the correlation coefficient for a backscatter measurement, anda₃-a_(n) may be the correlation coefficients for near angle forwardscatter measurements. The correlation coefficients a₁-a_(n) may bedeveloped using any suitable multivariate technique, including, forexample, a partial least squares technique. It is noted that thedependent variables x₁-x_(n) may relate to any measurement of a propertyof particulate matter, and should not be limited to the measurementassociations discussed above. In addition, there may be different setsof correlation coefficients for different fuels, which when combustedprovide different particle characteristics, and for different emissioncontrol equipment characteristics.

According to various embodiments, the correlation coefficients a₁-a_(n)may be developed in relation to a corresponding set of opticalmeasurements and mass loading measurements provided by a referencemethod or equivalent. The reference method may be, for example, themethod set forth at 40 C.F.R. Part 60, Appendix A, Reference Method 5 or17. A test set of simultaneous readings may be taken from the referencemethod and the various continuous methods, x₁-x_(n). The test set mayinclude readings taken over a range of process conditions. Correlationcoefficients a₁-a_(n), correlating the continuous methods to thereference method, may then be calculated from the test set using anymultivariate technique, including those discussed above. The test setmay take as input, x₁-x_(n), any combination of continuous methods thatmeasure a property of particulate matter, including, for example, theextinction, backscatter, and near angle forward scatter methodsdescribed above.

Equation 2 describes an alternative model that may be employed by thecorrelation module 712 to correlate various particulate matter propertymeasurements to mass loading.y(z ₁ , z ₂ , z ₃ , . . . z _(n))=a ₁ *z ₁(x ₁)+a ₂ *z ₂(x ₂)+a ₃ *z ₃(x₃)+ . . . +a _(n) *z _(n)(x _(n))  (2)

In equation 2, z₁-Z_(n) may represent nonlinear functions of thex₁-x_(n) dependent variables. For example, the functions z₁-Z_(n) mayhave polynomial, exponential, or logarithmic forms. A polynomialinstance of z₁ may be represented by equation 3 below.z ₁(x ₁)=b ₀ +b ₁ *x ₁ +b ₂ *x ₁ ² +b ₃ *x ₁ ³ +b ₄ *x ₁ ⁴ +b ₅ *x ₁⁵  (3)An exponential instance of z₁ may be represented by equation 4 below.z ₁(x ₁)=b ₁ *e ^(b0×1) +b ₂  (4)A logarithmic instance of z₁ may be represented by equation 5 below.z ₁(x ₁)=b ₁*ln(b ₂ x ₁ +b ₃)+b ₀  (5)It is noted that the coefficients b_(n) of the various instances of z₁described above need not be equal to the coefficients of z₂-Z_(n).Correlation coefficients a₁-a_(n) and the functions z₁-Z_(n) may befound by taking a test set similar to the one described above, and againperforming a correlation using any suitable multivariate method, such asfor example, partial least squares.

FIG. 9 is a flowchart of a process flow through the correlation module712 for developing a correlation model according to various embodimentsof the present invention. At block 902, the correlation module 712 orother computing devices not part of the monitoring system 700 mayreceive from the system 700 one or more test measurements of particulateproperties in the fluid 316. The test measurements may include, forexample, near forward scatter, backscatter, and extinction as describedabove, at different scattering angles and wavelengths of light. At block904, the correlation module 712 or other computing devices not part ofthe monitoring system 700 may receive as input the results of areference method measuring the mass loading of particulate present inthe fluid 316. In various embodiments, the reference method may also beimplemented by the system 700. The reference method results may beacquired at approximately the same time as the test measurements ofparticulate properties. The steps of blocks 902 and 904 may be repeatedas many times as desired to create a set of test data over a range ofoperating conditions. At block 906, the correlation module 712 or othercomputing devices not part of the monitoring system 700 may process datafrom the set of test data to calculate the correlation model. Thecorrelation model may take the form of Equations 1 or 2 above, forexample. The correlation model may be developed by, for example, solvingfor the correlation coefficients a₃-a_(n), and/or nonlinear equationsz₁-z_(n) of Equations 1 or 2 above.

FIG. 10 is a flowchart of a process flow through the correlation module712 for implementing a correlation model according to variousembodiments of the present invention. At block 1002, the correlationmodule 712 may receive from the system 700 one or more measurements ofparticulate properties in the fluid 316. At block 1004, the modeldeveloped above with reference to FIG. 9 may be applied by, for example,entering particulate properties into the Equations 1 or 2 as dependentvariables x₁-x_(n). Solving the equation may yield a measurement of massloading on a continuous basis.

In the description above, the near forward angle scatter devices 300were disclosed as directly measuring the particulate matter in the fluid316 in an exhaust stack 320. According to other embodiments, however,the near forward scatter device 300 could be used to measure theparticulate matter in a diverted portion of the exhaust stack 320, asshown in FIG. 11. In such an embodiment, a portion of the fluid 316 maybe diverted to a chamber 500. The device 300 may measure the particulatematter in the chamber 500, which may be indicative of the particulatematter mass loading in the fluid 316 in the exhaust stack 320. Such adiverted portion of the main fluid flow stream may be heated to vaporizeany entrained water droplets, or otherwise arranged to accommodate veryhigh temperature gas streams or very large or small stack/ductdiameters, or to pull the gas stream to a more convenient or protectedmeasurement environment. Isokinetic sampling of the main stream may benecessary if a representative particulate sample of the main gas streamis to be obtained.

Also, according to other embodiments in the system 700 for takingdifferent types of optical measurements, instead of using a wheel 706 tomove the various reflectors/absorbers into the path of the beam, one ora number of swing arms 1204 may be used, as shown in FIG. 12. The one ormore swing arms 1204 may be operated by a stepper motor or otherelectromechanical positioning device (not shown) which may be controlledby the computing device 710. A near angle forward reflector 310, anextinction reflector 110 or an absorbing device 218 may be mounted oneach swing arm 1204 and lowered into the path of the beam when the swingarm 1204 is lowered. According to various other embodiments, variousother optical devices, for example, a front portion 318, or abackscatter absorbing device 218, may be mounted on swing arms 1204.Lowering the swing arms 1204 may cause one or more of the various otheroptical devices to be placed in front of one stationary reflector 310.

As shown in FIG. 13, the system 700 may be calibrated by placing one ormore optical calibration devices 1302, 1304, which may be, for example,filters of varying reflectivity, into the path of the beams 312, 314.The optical calibration devices 1302, 1304 may simulate known conditionswhere a certain amount of particulate is present. For example, if anoptically black filter is used as one of the calibration devices 1302,1304, then none of the light from the light source should reach thetransceiver 301, simulating a scatter measurement with no particulatepresent. Conversely, if a somewhat reflective filter is used as one ofthe calibration device 1302, 1304, then a substantial portion of thelight may reach the transceiver 301, simulating an extinctionmeasurement with particulate present. Other filters with intermediatereflectivity may also be used to simulate other conditions.

Optical calibration devices 1302, 1304 may be placed into the path ofbeams 312, 314 by a variety of methods. For example, the opticalcalibration devices 1302, 1304 may be mounted on swing arms andmanipulated into the beam path by the same method described above inconjunction with reflector 1202 and swing arm 1204. Also, opticalcalibration devices 1302, 1304 may be mounted on a calibration plate1306 as shown in FIGS. 13. The calibration plate 1306 may be rotatedinto the beam path by a stepper motor or other electromechanicalpositioning device (not shown) in a manner similar to that describedabove with reference to wheel 706.

While several embodiments of the invention have been described, itshould be apparent that various modifications, alterations andadaptations to those embodiments may occur to persons skilled in theart. For example, various particulate matter measurement methods may beadded or subtracted. It is therefore intended to cover all suchmodifications, alterations and adaptations without departing from thescope and spirit of the present invention as defined by the appendedclaims.

1. A device for measuring near forward scatter of light caused byparticulate matter in a fluid, the device comprising: a transceiver, thetransceiver comprising: a light source for projecting a light beamthrough the fluid; and a detector positioned on the same side of thefluid as the light source; and a reflector positioned opposite thetransceiver so that at least a portion of the fluid is present betweenthe transceiver and the reflector, wherein the reflector includes afront portion facing the transceiver, such that optical energy from thelight source of the transceiver incident upon the reflector is reflectedtowards the transceiver as a return beam of optical energy, whereinprimarily optical energy of the return beam that is scattered over arange of near forward angles by particulate matter in the fluid reachesthe detector.
 2. The device of claim 1, wherein the range of nearforward angles is between 1 and 5 degrees.
 3. The device of claim 1,wherein the front portion of the reflector includes a lens.
 4. Thedevice of claim 3, wherein the front portion of the reflector includes aconvex lens including a non-transmissive central portion.
 5. The deviceof claim 3, wherein the front portion of the reflector includes aconcave wedge shaped lens.
 6. The device of claim 5, wherein the concavewedge shaped lens includes a non-transmissive central portion.
 7. Thedevice of claim 3, wherein the front portion of the reflector includes aconvex wedge shaped lens.
 8. The device of claim 7, wherein the convexwedge shaped lens includes a non-transmissive central portion.
 9. Thedevice of claim 1, wherein the transceiver further comprises a secondlight source with a wavelength different than a wavelength of the firstlight source.
 10. The device of claim 3, wherein the lens includes anon-transmissive central portion.
 11. The device of claim 3, furthercomprising means for replacing the lens of the reflector with a second,differently shaped lens.
 12. The device of claim 3, further comprisingmeans for replacing the reflector with a second reflector.
 13. Thedevice of claim 12, wherein the second reflector includes a frontportion facing the transceiver, such that optical energy from the lightsource of the transceiver incident upon the second reflector isreflected towards the transceiver as a return beam of optical energy,wherein primarily optical energy of the return beam that is scatteredover a second range of near forward angles by particulate matter in thefluid reaches the detector.
 14. The device of claim 1, wherein thereturn beam crosses itself in the field of view of the detector.
 15. Thedevice of claim 1, wherein the light source is selected from the groupconsisting of a light emitting diode, a laser diode, and an incandescentsource.
 16. The device of claim 1, wherein the device is mounted on anexhaust stack and is for measuring particulate matter present in anexhaust stream in the exhaust stack.
 17. The device of claim 1, whereinthe device is mounted in a diversion chamber and is for measuringparticulate matter present in a fluid that has been diverted from anexhaust stream into the diversion chamber.
 18. A system for monitoringparticulate matter in a fluid comprising: a transceiver, comprising: alight source for projecting a light beam through the fluid; and adetector; and; a reflector assembly positioned opposite the transceiverso that at least a portion of the fluid is present between thetransceiver and the reflector assembly, the reflector assemblycomprising: a first reflector including a front portion facing thetransceiver, such that, when the first reflector is in the path of thelight beam from the light source, optical energy from the light sourceincident upon the reflector is reflected towards the transceiver as areturn beam of optical energy, wherein primarily optical energy of thereturn beam that is scattered over a range of near forward angles byparticulate matter in the fluid; a second optical device; and means forcyclically moving the first reflector and the second optical device intothe path of the light beam from the light source.
 19. The system ofclaim 18, wherein the second optical device includes an extinctionreflector.
 20. The system of claim 18, wherein the reflector assemblyfurther includes an absorbing device, and wherein the means forcyclically moving is further for cyclically moving the first reflector,the second optical device and the absorbing device into the path of thelight beam from the light source.